A partial perspective view of an AC plasma display panel (hereinafter referred to as a "panel") is shown in FIG. 4. As shown in FIG. 4, a scanning electrode 4 and a sustain electrode 5 that are covered with a dielectric layer 2 and a protective film 3 are provided on a first glass substrate 1 in parallel with each other as a pair. On a second glass substrate 6, a plurality of data electrodes 8 covered with an insulator layer 7 are provided. Separation walls 9 are provided in parallel to the data electrodes 8 on the insulator layer 7 between every two of the data electrodes 8. Phosphors 10 are formed on the surface of the insulator layer 7 and on both side faces of each separation wall 9. The first glass substrate 1 and the second glass substrate 6 are positioned opposing each other with discharge spaces 11 being sandwiched therebetween so that the scanning electrodes 4 and the sustain electrodes 5 are orthogonal to the data electrodes 8. In the discharge spaces 11, xenon and at least one selected from helium, neon, and argon are filled as discharge gases. The discharge spaces at the intersections of the data electrodes 8 and pairs of scanning electrode 4 and sustain electrode 5 form respective discharge cells 12.
FIG. 5 is a diagram showing the electrode array in this panel. As shown in FIG. 5, this electrode array has a matrix structure formed of m columns.times.n rows. In the column direction, m columns of data electrodes D.sub.1 -D.sub.m are arranged, and n rows of scanning electrodes SCN.sub.1 -SCN.sub.n and sustain electrodes SUS.sub.1 -SUS.sub.n are arranged in the row direction. The discharge cell 12 shown in FIG. 4 corresponds to the region shown in FIG. 5.
FIG. 6 is a diagram showing the timing chart of an operation driving waveform in a conventional driving method for driving this panel. This driving method is used for displaying 256 shades of gray. One field consists of eight subfields. This driving method is described with reference to FIGS. 4 to 6 as follows.
As shown in FIG. 6, each of first to eighth subfields includes an initialization period, a write period, a sustain period, and an erase period. First, the description is directed to the operation in the first subfield.
As shown in FIG. 6, all the data electrodes D.sub.1 -D.sub.m and all the sustain electrodes SUS.sub.1 -SUS.sub.n are maintained at a voltage of 0 in an initialization operation in a first part of the initialization period. To all the scanning electrodes SCN.sub.1 -SCN.sub.n, a lamp voltage is applied, which increases gradually from a voltage of VP toward a voltage of Vr. The voltages of Vp and Vr provide the scanning electrodes SCN.sub.1 -SCN.sub.n with voltages below and beyond the discharge starting voltage with respect to the sustain electrodes SUS.sub.1 -SUS.sub.n, respectively. During the lamp voltage increases, a first weak initialization discharge occurs in all the discharge cells 12 from the scanning electrodes SCN.sub.1 -SCN.sub.n to the data electrodes D.sub.1 -D.sub.m and the sustain electrodes SUS.sub.1 -D.sub.n, respectively. Due to the first weak initialization discharge, a negative wall voltage is stored in the regions of the protective film 3 surface that are positioned on the scanning electrodes SCN.sub.1 -SCN.sub.n (hereinafter this terminology is described simply as "at the surface of the protective film 3 on the scanning electrodes SCN.sub.1 -SCN.sub.n "). At the same time, a positive wall voltage is stored at the surface of insulator layer 7 on the data electrodes D.sub.1 -D.sub.m and at the surface of the protective film 3 on the sustain electrodes SUS.sub.1 -SUS.sub.n.
In the initialization operation in a second part of the initialization period, all the sustain electrodes SUS.sub.1 -SUS.sub.n are maintained at a positive voltage of Vh. To all the scanning electrodes SCN.sub.1 -SCN.sub.n, a lamp voltage is applied, which decreases gradually from a voltage of Vq toward a voltage of 0. The voltages of Vq and 0 provide the scanning electrodes SCN.sub.1 -SCN.sub.n with voltages below and beyond the discharge starting voltage with respect to the sustain electrodes SUS.sub.1 -SUS.sub.n, respectively. During the lamp voltage decreases, a second weak initialization discharge occurs again in all the discharge cells 12 from the sustain electrodes SUS.sub.1 -SUS.sub.n to the scanning electrodes SCN.sub.1 -SCN.sub.n. The second weak initialization discharge weakens the negative wall voltage at the surface of the protective film 3 on the scanning electrodes SCN.sub.1 -SCN.sub.n and the positive wall voltage at the surface of the protective film 3 on the sustain electrodes SUS.sub.1 -SUS.sub.n. A weak discharge also occurs between the data electrodes D.sub.1 -D.sub.m and the scanning electrodes SCN.sub.1 -SCN.sub.n. Consequently, the positive wall voltage at the surface of the insulator layer 7 on the data electrodes D.sub.1 -D.sub.m is adjusted to a value suitable for a write operation.
Thus, the initialization operation in the initialization period is completed.
In the write operation in the subsequent write period, initially all the scanning electrodes SCN.sub.1 -SCN.sub.n are maintained at a voltage of Vs. Then, a positive write pulse voltage of +Vw is applied to a designated data electrode D.sub.j (j indicates one or more integers of 1 to m) that is selected from the data electrodes D.sub.1 -D.sub.m and corresponds to a discharge cell 12 to be operated so as to emit light in the first line and at the same time a scan pulse voltage of 0 is applied to the scanning electrode SCN.sub.1 of the first line. In this state, the voltage between the surface of the insulator layer 7 and the surface of the protective film 3 on the scanning electrode SCN.sub.1 at the intersection of the designated data electrode D.sub.j and the scanning electrode SCN.sub.1 is calculated by adding the positive wall voltage at the surface of the insulator layer 7 on the data electrodes D.sub.1 -D.sub.m to the write pulse voltage of +Vw. Therefore, at this intersection, a write discharge occurs be tween the designated data electrode D.sub.j and the scanning electrode SCN.sub.1 and between the sustain electrode SUS.sub.1 and the scanning electrode SCN.sub.1. Thus, at this intersection, a positive wall voltage is stored at the surface of the protective film 3 on the scanning electrode SCN.sub.1, a negative wall voltage at the surface of the protective film 3 on the sustain electrode SUS.sub.1, and a negative wall voltage at the surface of the insulator layer 7 on the data electrode D.sub.j.
Then, a positive write pulse voltage of +Vw is applied to a designated data electrode D.sub.j that is selected from the data electrodes D.sub.1 -D.sub.m and corresponds to a discharge cell 12 to be operated so as to emit light in the second line. At the same time, a scan pulse voltage of 0 is applied to the scanning electrode SCN.sub.2 of the second line. In this state, the voltage between the surface of the insulator layer 7 and the surface of the protective film 3 on the scanning electrode SCN.sub.2 at the intersection of the designated data electrode D.sub.j and the scanning electrode SCN.sub.2 is calculated by adding the positive wall voltage stored at the surface of the insulator layer 7 on the designated data electrode D.sub.j to the write pulse voltage of +Vw. Therefore, at this intersection, a write discharge occurs between the designated data electrode D.sub.j and the scanning electrode SCN.sub.2 and between the sustain electrode SUS.sub.2 and the scanning electrode SCN.sub.2. As a result, at this intersection, a positive wall voltage is stored at the surface of the protective film 3 on the scanning electrode SCN.sub.2, a negative wall voltage at the surface of the protective film 3 on the sustain electrode SUS.sub.2, and a negative wall voltage at the surface of the insulator layer 7 on the data electrode D.sub.j.
Successively, the same operation is carried out for all remaining lines. Finally, a positive write pulse voltage of +Vw is applied to a designated data electrode D.sub.j that is selected from the data electrodes D.sub.1 -D.sub.m and corresponds to s discharge cell 12 to be operated so as to emit light in the nth line. At the same time, a scan pulse voltage of 0 is applied to a scanning electrode SCN.sub.n of the nth line. This causes write discharges between the designated data electrode D.sub.j and the scanning electrode SCN.sub.n and between a sustain electrode SUS.sub.n and the scanning electrode SCN.sub.n at the intersection of the designated data electrode D.sub.j and the scanning electrode SCN.sub.n. As a result, at this intersection, a positive wall voltage is stored at the surface of the protective film 3 on the scanning electrode SCN.sub.n, a negative wall voltage at the surface of the protective film 3 on the sustain electrode SUS.sub.n, and a negative wall voltage at the surface of the insulator layer 7 on the data electrode D.sub.j.
Thus, the write operation in the write period is completed.
In the subsequent sustain period, the voltage of all the scanning electrodes SCN.sub.1 -SCN.sub.n and all the sustain electrodes SUS.sub.1 -SUS.sub.n is restored to 0 for the time being. After that, initially a positive sustain pulse voltage of +Vm is applied to all the scanning electrodes SCN.sub.1 -SCN.sub.n. In this state, the voltage between the surface of the protective film 3 on a scanning electrode SCN.sub.i (i indicates one or more integers of 1 to n) in the discharge cell 12 in which the write discharge has occurred and the surface of the protective film 3 on the sustain electrodes SUS.sub.1 -SUS.sub.n is calculated by adding the positive wall voltage stored at the surface of the protective film 3 on the scanning electrode SCN.sub.i and the negative wall voltage stored at the surface of the protective film 3 on a sustain electrode SUS.sub.i, which have been stored in the write period, to the sustain pulse voltage of +Vm and thus exceeds the discharge starting voltage. Therefore, in the discharge cell in which the write discharge has occurred, a sustain discharge occurs between the scanning electrode SCN.sub.i and the sustain electrode SUS.sub.i. In the discharge cell in which the sustain discharge has occurred, a negative wall voltage is stored at the surface of the protective film 3 on the scanning electrode SCN.sub.i, and a positive wall voltage is stored at the surface of the protective film 3 on the sustain electrode SUS.sub.i. After that, the sustain pulse voltage applied to the scanning electrodes SCN.sub.1 -SCN.sub.n is restored to 0.
Successively, a positive sustain pulse voltage of +Vm is applied to all the sustain electrodes SUS.sub.1 -SUS.sub.n. In this state, in the discharge cell in which the sustain discharge has occurred, the voltage between the surface of the protective film 3 on the sustain electrode SUS.sub.i and the surface of the protective film 3 on the scanning electrode SCN.sub.i is calculated by adding the negative wall voltage at the surface of the protective film 3 on the scanning electrode SCN.sub.i and the positive wall voltage at the surface of the protective film 3 on the sustain electrode SUS.sub.i, which have been stored by the preceding sustain discharge, to the sustain pulse voltage of +Vm. Therefore, in the discharge cell in which this sustain discharge has occurred, a sustain discharge occurs between the sustain electrode SUS.sub.i and the scanning electrode SCN.sub.i. Thus, in this discharge cell, a negative wall voltage is stored at the surface of the protective film 3 on the sustain electrode SUS.sub.i and a positive wall voltage is stored at the surface of the protective film 3 on the scanning electrode SCN.sub.i. After that, the sustain pulse voltage is restored to 0.
Hereafter in the same way as mentioned above, a positive sustain pulse voltage of +Vm is applied to all the scanning electrodes SCN.sub.1 -SCN.sub.n and all the sustain electrodes SUS.sub.1 -SUS.sub.n alternately, thus causing a continuous sustain discharge. At the conclusion of the sustain period, a positive sustain pulse voltage of +Vm is applied to all the scanning electrodes SCN.sub.1 -SCN.sub.n. In this state, in the discharge cell in which the sustain discharge has occurred, the voltage between the surface of the protective film 3 on the scanning electrode SCN.sub.i and the surface of the protective film 3 on the sustain electrode SUS.sub.i is calculated by adding the positive wall voltage at the surface of the protective film 3 on the scanning electrode SCN.sub.i and the negative wall voltage at the surface of the protective film 3 on the sustain electrode SUS.sub.i, which have been stored by the preceding sustain discharge, to the sustain pulse voltage of +Vm. Therefore, in the discharge cell in which this sustain discharge has occurred, a sustain discharge occurs between the scanning electrode SCN.sub.i and the sustain electrode SUS.sub.i. Thus, in this discharge cell, a negative wall voltage is stored at the surface of the protective film 3 on the scanning electrode SCN.sub.i and a positive wall voltage is stored at the surface of the protective film 3 on the sustain electrode SUS.sub.i. After that, the sustain pulse voltage is restored to 0. Thus, the sustain operation in the sustain period is completed. Visible emission from the phosphors 10 excited by ultraviolet rays generated by this sustain discharge is used for display.
In the subsequent erase period, a lamp voltage that increases gradually from a voltage of 0 toward +Ve is applied to all the sustain electrodes SUS.sub.1 -SUS.sub.n. In this state, in the discharge cell in which the sustain discharge has occurred, the voltage between the surface of the protective film 3 on the scanning electrode SCN.sub.i and the surface of the protective film 3 on the sustain electrode SUS.sub.i is calculated by adding a negative wall voltage at the surface of the protective film 3 on the scanning electrode SCN.sub.i and a positive wall voltage at the surface of the protective film 3 on the sustain electrode SUS.sub.i at the conclusion of the sustain period, to this lamp voltage. Therefore, in the discharge cell in which the sustain discharge has occurred, a weak erase discharge occurs between the sustain electrode SUS.sub.i and the scanning electrode SCN.sub.i, and therefore the negative wall voltage at the surface of the protective film 3 on the scanning electrode SCN.sub.i and the positive wall voltage at the surface of the protective film 3 on the sustain electrode SUS.sub.i are weakened, thus terminating the erase discharge.
Thus, the erase operation in the erase period is completed.
In the above operations, as to the discharge cells that are not operated to emit light, the initialization discharge occurs in the initialization period, but the write discharge, the sustain discharge, and the erase discharge do not take place. Therefore, in the discharge cells that are not operated to emit light, the wall voltage stored at the surface of the protective film 3 on the scanning electrode SCN.sub.i and the sustain electrode SUS.sub.i and the wall voltage stored at the surface of the insulator layer 7 on the data electrode D.sub.h (h indicates one or more integers of 1 to n, which is not the same as j) are maintained at the levels when the initialization period was completed.
By all the operations described above, one picture in the first subfield is displayed. The same operations are carried out over the second to the eighth subfields. The luminance of the discharge cells that are operated to emit light in these subfields is determined depending on how many times the sustain pulse voltage of +Vm is applied. Therefore, for instance, by suitably setting the number of times of the application of the sustain pulse voltage in each subfield so that one field consists of eight subfields whose relative magnitudes of the luminance obtained by the sustain discharge are 2.sup.0, 2.sup.1, 2.sup.2, . . . 2.sup.7, a display having 2.sup.8 =256 shades of gray can be obtained.
According to the conventional driving method described above, in the display of a so-called "black picture" in which no discharge cell is in a display state, the write discharge, the sustain discharge, and the erase discharge do not occur and only the initialization discharge occurs. This initialization discharge is weak and its discharge emission also is weak. Therefore, this driving method is characterized by a high contrast in a panel. For example, when the 256 shades of gray were displayed using a structure in which each field consists of eight subfields in a 42-inch AC plasma display panel having a matrix structure formed of 480 rows and 852.times.3 columns, the emission luminance obtained by the first and second initialization discharges in the initialization period in each subfield was 0.15 cd/m.sup.2. Therefore, the sum of the emission luminance in the eight subfields is 0.15.times.8=1.2 cd/m.sup.2. Since the maximum luminance is 420 cd/m.sup.2, the contrast in this panel is 420/1.2:1=350:1. Thus, a quite high contrast can be obtained.
As described above, in the above-mentioned conventional driving method, a quite high contrast can be obtained when the panel display is carried out under a normal lighting condition. However, since an initialization discharge occurs twice in each subfield without exception, even the emission caused by these weak initialization discharges has luminance so high as to be noticeable when the panel display is carried out in dark surroundings. Therefore, when the panel display is carried out in a place where it is not so bright, poor visibility of the black display has been a problem.